In order to control emissions from an internal combustion engine and its efficiency, it is necessary to control the fuel delivered into the cylinder of the internal combustion engine with a high degree of accuracy. For example, if too much fuel is supplied into the cylinder, the fuel is not burned properly and is wasted. As a result, the efficiency of the engine suffers. If too little fuel is supplied into the cylinder, the power of the engine is not fully utilized. If the amount of fuel supplied into the cylinder of the engine is held within close tolerances (for example, within one (1) percent of a desired value) the efficiency of the engine is increased and the amount of harmful emissions is reduced.
In fuel supply means wherein the fuel is injected into the cylinder of an internal combustion engine, in metered or measured amounts, the viscosity of the liquid fuel determines the time required to meter a predetermined desired volume of the liquid fuel. In other words, the time required for a predetermined volume to flow varies with viscosity. In the case of a liquid flowing through a long tube of small diameter, the volume V of liquid which escapes in a time t is given by the equation: ##EQU1## where p is the pressure between the two ends of the tube; r its radius; l its length and n the viscosity (Law of Poiseuille). An electronic fuel injector which injects fuel into a cylinder of an internal combustion engine is controlled by a metering solenoid which controls the amount of liquid fuel flowing through a metering valve. The longer the metering solenoid is actuated, the greater the amount of fuel which flows through the metering valve of the fuel injector. Given a fixed period of time, a smaller amount of fuel having a high viscosity will flow through the metering valve than the amount of fuel having a lower viscosity.
The viscosity of a liquid fuel is given by the equation: ##EQU2## where d is the density in g/cm.sup.2 ; r, the radius, l the length of the tube in cm; Q the volume in cm.sup.3 discharged in t sec; .lambda. a correction to the length of the tube; h, the average head in cm; in the expression mv.sup.2 /g, m is the coefficient of the kinetic energy correction; g is the acceleration due to gravity in cm/sec.sup.2, and v is the mean velocity in cm/sec. The volume of fuel delivered in a given time is a function of viscosity as shown by the first equation. In order to deliver a given volume, regardless of variation in viscosity, the time period of flow of the metered charge may be varied as a function of viscosity. The viscosity of a liquid is related to the mass or BTU content of the fuel delivered since the viscosity is a function of the fuel density as shown by the second equation.
If the actuation time of the metering solenoid is to be controlled to provide a predetermined or desired metered amount or volume of liquid fuel to the cylinder of an internal combustion engine, it is necessary to know the viscosity of the liquid fuel. This is particularly true in the case of a diesel engine which can operate on liquid fuel of varying viscosity, for example, No. 1 or No. 2 diesel fuel.
Prior patents disclose ways of regulating the volume or rate of flow of liquid fuel. For example, the U.S. Pat. No. 2,996,053 to Evans discloses a fuel control system in which the maximum rate of delivery of a fuel pump is regulated according to the density of the fuel. Fuel density is determined by the position of a float in a fuel chamber. Alternatively, fuel density is determined by the position of a spring mounted cup which holds a given quantity of fuel. A rod connected with the float or cup positions a wedge which acts as an abutment for a fuel regulation rod to control the quantity of fuel delivered by the pump in accordance with the density.
The U.S. Pat. No. 3,307,391 to Parker discloses a fuel control system in which the viscosity of the fuel is continuously measured. In accordance with the viscosity measurement, the maximum volumetric rate at which the fuel is being used is limited. The viscosity is measured by causing the fuel to flow at a constant rate into a chamber and allowing it to escape through an orifice so that the outflow is inversely proportional to the viscosity. The pressure within the chamber is taken as an indication of viscosity and a fuel control lever is positioned according to the measured value of pressure.
The French Patent to Angeli et al 882,148, discloses a fuel injection system in which the volume of fuel injected is regulated in accordance with the temperature of the fuel. A temperature sensitive bulb controls a cam position which, in turn, alters the effective length of a fuel control rod of the injector pump.
The U.S. Pat. No. 3,483,855 to Thoma discloses an arrangement in which the volume of fuel injected is regulated in accordance with the vapor pressure of the fuel.